1. Field of the Invention
The invention relates to metal halide lamps, and more specifically, to improving the color rendition and luminous output of sodium-scandium metal halide lamps.
2. Description of the Prior Art
Metal halide lamps have been known and manufactured for approximately 30 years and typically include an inner quartz arc tube containing a fill of arc-sustaining material and surrounded by an outer glass envelope. The fill of the arc tube includes a rare gas for starting and a quantity of mercury. The lamp's emission spectrum is primarily due to the presence in the arc tube fill of one or more metal halides, usually iodides. The luminous efficacy, color rendering index, and other lamp output characteristics may be varied by the selection of the particular composition of the metal halides in the arc tube fill. For example, U.S. Pat. No. 3,234,421 to Reiling generally discloses metal halide lamps including iodides selected from those of Li, Na, Cs, Ca, Cd, Ba, Hg, Ga, In, Tl, Ge, Sn, Th, Se, Te and Zn.
In the United States, lamps based on a metal halide fill of predominantly sodium and scandium have been commercially successful due to their very good luminous efficacy and long operating life. Recently, low-wattage metal halide lamps (generally 100 watts or less) have found wider application as a replacement for incandescent lamps for general interior lighting and display lighting. In these applications, it is desirable to have good color rendering as well as high efficacy. An efficacy of greater than about 75 LPW and a color rendering R.sub.a greater than about 65 would be particularly advantageous for indoor low-wattage use. Additionally, it is desirable that the color temperature of such a metal halide lamp be as close to an incandescent lamp (CCT.congruent.2850) as is practicable while maintaining the efficacy and CRI above 75 LPW and 65, respectively. However, metal halide lamps with the desired characteristics are not available in the prior art.
U.S. Pat. No. 3,979,624 to Liu et al teaches that enhanced efficacy in a lamp containing sodium and/or lithium iodides, as well as scandium iodide, requires a low alkali metal to scandium ratio of below 5:1. This is in contrast to the lamps of U.S. Pat. No. 3,407,327 to Corey et al which shows an NaI:ScI.sub.3 molar ratio in excess of 11.5.
U.S. Pat. No. 4,247,798 to Howe et al shows a conventional tubular quartz arc tube metal halide lamp having lithium iodide to increase the proportion of red radiation and enhance color-rendering in a sodium-scandium lamp. In Howe, the molar proportion of lithium iodide is in the range of 10 to 50 percent of the total of sodium, lithium, and scandium iodides and the total molar ratio of alkali metal iodides to scandium iodide is between 5.4:1 and 57.1:1. Howe discloses lamps having efficacies of 66-76 lumens per watt (LPW) with corresponding color rendering indices R.sub.a of 84-74. Howe's data shows that the addition of any quantity of lithium iodide reduces the luminous efficacy. The Howe lamp achieved improved color rendition only at the expense of reductions in luminous efficacy as compared to a sodium scandium lamp without lithium iodide. Moreover, the Howe lamps had relatively high correlated color temperatures (CCT) of 3900.degree.-4500.degree. K. High color temperatures, for example, above about 4000.degree. K. are undesirable for many applications because they provide a bluer or "colder" color than incandescent lamps, which are considered "warm".
U.S. Pat. No. 4,866,342 to Ramaiah et al discloses a tubular arc tube sodium-scandium lamp which includes thallium halide in a mole ratio of sodium halide to thallium halide of between 280:1 to 75:1 to increase the luminous efficacy of the lamp. With small amounts of thallium iodide, corresponding to the preferred NaI/TlI range of 260:1 to 240:1, both the color rendering and the efficacy are improved. With larger amounts of TlI the efficacy continues to increase but only at the expense of reductions in the color rendering. With small quantities of thallium corresponding to NaI/TlI ratios above 280:1 there is insufficient thallium to cause any appreciable increase in luminous efficacy. Ramaiah discloses lamps having relatively high luminous efficacies of between 85.4 and 97.3 LPW but with relatively low color rendering indexes R.sub.a of between about 62 and 55. The high luminous efficacies were only achieved with high CCT's of between 3850.degree. and 4500.degree. K.
Because of its wide commercial acceptance, it would be desirable to further improve upon the sodium-scandium metal halide lamp, and particularly for low-wattage applications.
Accordingly, it is an object of the invention to improve the color rendition of a sodium-scandium metal halide lamp while increasing its efficacy.
Another object of the invention is to provide such a lamp which also has a CCT below that of the prior art lamps.
Yet another object of the invention is to provide a sodium-scandium metal halide lamp having a luminous efficacy of greater than about 75 LPW and a color rendering index R.sub.a of greater than about 65.
Still another object of the invention is to provide such a lamp which also has a correlated color temperature below about 3500.degree..